2.0 SOIL SCREENING PROCESS
Developing the CSM involves several steps, discussed in the following subsections.
The soil screening process (Exhibit 4) is a step-by-step approach that involves: + Developing a conceptual site model (CSM) + Comparing the CSM to the SSL scenario + Defining data collection needs + Sampling and analyzing soils at site + Calculating site-specific SSLs + Comparing site soil radionuclide concentrations to calculated SSLs + Determining which areas of the site require further study. It is important to follow this process to implement the Soil Screening Guidance for Radionuclides properly. The remainder of this guidance discusses each activity in detail.
2.1.1 Collect Existing Site Data. The initial design of the CSM is based on existing site data compiled during previous studies. These data may include site sampling data, historical records, aerial photographs, maps, and State soil surveys, as well as information on local and regional conditions relevant to radionuclide migration and potential receptors. Data sources include Superfund site assessment documents (i.e., the PA/SI), documentation of removal actions, and records of other site characterizations or actions. Published information on local and regional climate, soils, hydrogeology, and ecology may be useful. In addition, information on the population and land use at and surrounding the site will be important to identify potential exposure pathways and receptors. The RI/FS guidance (U.S. EPA, 1989c) discusses collection of existing data during RI scoping, including an extensive list of potential data sources. The Multi-Agency Radiation Survey and Site Investigation Manual (MARSSIM) (U.S. EPA, 1997b) [Section 3.4] discusses the collection of existing data specific to sites contaminated with radioactive materials. 2.1.2 Organize and Analyze Existing Site Data. One of the most important aspects of the CSM
development process is to identify and characterize all potential exposure pathways and receptors at the site by considering site conditions, relevant exposure scenarios, and the properties of radionuclides present in site soils. Attachment A, the Conceptual Site Model Summary, provides four forms for organizing site data for soil screening purposes. The CSM summary organizes site data according to general site information, soil radionuclide source characteristics, exposure pathways and receptors. Note: If a CSM has already been developed for the site in question, use the summary forms in Attachment A to ensure that it is adequate.
2.1
Step 1: Developing a Conceptual Site Model
The CSM is a three-dimensional “picture” of site conditions that illustrates radionuclide distributions, release mechanisms, exposure pathways and migration routes, and potential receptors. The CSM documents current site conditions and is supported by maps, cross sections, and site diagrams that illustrate human and environmental exposure through radionuclide release and migration to potential receptors. Developing an accurate CSM is critical to proper implementation of the Soil Screening Guidance for Radionuclides. As a key component of the RI/FS and EPA’s Data Quality Objectives (DQO) process, the CSM should be updated and revised as investigations produce new information about a site. Data Quality Objectives for Superfund: Interim Final Guidance (U.S. EPA, 1993a) and Guidance for Conducting Remedial Investigations and Feasibility Studies under CERCLA (U.S. EPA, 1989c) provide a general discussion about the development and use of the CSM during RIs.
2-1
Exhibit 4 Soil Screening Process for Radionuclides
Step One: Develop Conceptual Site Model • Collect existing site data (historical records such as previous surveys and sampling data, site operating records, aerial photographs, maps, PA/SI data, available background information, State soil surveys, etc.) • Organize and analyze existing site data - Identify known sources of contamination and potential contaminants - Identify potentially contaminated areas and affected media - Identify potential migration routes, exposure pathways, and receptors • Construct a preliminary diagram of the CSM • Perform site reconnaissance - Confirm and/or modify CSM - Identify remaining data gaps Compare Soil Component of CSM to Soil Screening Scenario • Confirm that future residential land use is a reasonable assumption for the site • Identify pathways present at the site that are addressed by the guidance • Identify additional pathways present at the site not addressed by the guidance • Compare pathway-specific generic SSLs with available concentration data • Estimate whether background levels exceed generic SSLs Define Data Collection Needs for Soils to Determine Which Site Areas Exceed SSLs • Stratify the site based on existing data • Identify exposure areas • Develop sampling and analysis plan for determining mean soil radionuclide concentrations - Determine appropriate survey instruments and techniques and establish QA/QC protocols - Sampling strategy for surface soils (includes defining study boundaries, developing a decision rule, specifying limits on decision errors, and optimizing the design) - Sampling strategy for subsurface soils (includes defining study boundaries, developing a decision rule, specifying limits on decision errors, and optimizing the design) - Sampling to measure soil characteristics (bulk density, moisture content, porosity, soil texture, pH) • Determine appropriate field methods and establish QA/QC protocols Sample and Analyze Soils at Site • Identify radionuclides • Delineate area and depth of sources and identify non-impacted areas as appropriate • Determine soil characteristics • Conduct preliminary data review • Revise CSM, as appropriate Derive Site-specific SSLs, if needed • Identify SSL equations for relevant pathways • Obtain site-specific input parameters from CSM summary • Replace variables in SSL equations with site-specific data gathered in Step 4 • Calculate SSLs - Account for exposure to multiple contaminants Compare Site Soil Contaminant Concentrations to Calculated SSLs • Select appropriate statistical tests and verify test assumptions • For surface soils, screen out exposure areas where all composite samples do not exceed SSLs by a factor of two • For subsurface soils, screen out source areas where the highest average soil core concentration does not exceed the SSLs Decide How to Address Areas Identified for Further Study • Review and confirm the data that led to the decision • Consider likelihood that additional areas can be screened out by collecting additional data • Integrate soil data with other media in the baseline risk assessment to estimate cumulative risk at the site • Determine the need for action • Use SSLs as PRGs
Step Two:
Step Three:
Step Four:
Step Five:
Step Six:
Step Seven:
2-2
2.1.3 Construct a Preliminary Diagram of the CSM. Once the existing site data
have been organized and a basic understanding of the site has been attained, draw a preliminary “sketch” of the site conditions, highlighting source areas, potential exposure pathways, and receptors. Ultimately, when site investigations are complete, this sketch will be refined into a three-dimensional diagram that summarizes the data. Also, a brief summary of the contamination problem should accompany the CSM. Attachment A provides an example of a complete CSM summary.
potential pathways of exposure to radioactive soil contaminants in a residential setting and are addressed by this guidance document: + Direct ingestion of soil + Inhalation of fugitive dusts + Ingestion of contaminated ground water caused by migration of radionuclides through soil to an underlying potable aquifer + External radiation exposure from radionuclides in soil + Ingestion of homegrown produce that has been contaminated via plant uptake This guidance quantitatively addresses each of these pathways. Whether some or all of the pathways are relevant at the site depends upon the radionuclides and conditions at the site. For surface soils under the residential land use assumption, the external exposure pathway will typically be the dominant exposure pathway for most radionuclides (e.g., 54Mn, 60Co, 137Cs, etc.). For some radionuclides (e.g., 3H, 99Tc, 129I, etc.), the ground water pathway often dominates, although not to the extent that the external exposure pathway does. The plant ingestion pathway and soil ingestion pathway also play a dominant role for a few radionuclides of interest (for plant ingestion - 14C, 63Ni, 90Sr, etc.; for soil ingestion 241 Am, 244Cm, 230Th, 232Th, etc.). In the majority of cases, the inhalation of fugitive dust pathway plays an insignificant role. For subsurface soils, risks from migration of radionuclides to an underlying aquifer is the only potential concern for this scenario. Volatilization is not included as a pathway since it is a concern for only a very limited number of radionuclides (such as 3H and 14 C). The majority of all radionuclides are present in soil as nonvolatile ionic species or inorganic compounds (i.e., Henry’s law constant is zero). Thus, volatilization and subsequent inhalation has not been included. If 3H or 14C volatilization is a concern, an approach similar to that in the Soil Screening Guidance for chemicals can be used to model the exposure. Consideration of the ground water pathway may be eliminated if ground water beneath or adjacent to the site is not a potential source of drinking water. Coordinate this decision on a site-specific basis with State or local authorities responsible for ground water use and classification. The
2-3
2.1.4
Perform Site Reconnaissance. At
this point, a site visit would be useful because conditions at the site may have changed since the PA/SI was performed (e.g., removal actions may have been taken). During si t e r econnaissance, update site sketches/topographic maps with the locations of buildings, source areas, wells, and sensitive environments. Anecdotal information from nearby residents or site workers may reveal undocumented disposal practices and thus previously unknown areas of contamination that may affect the current CSM interpretation. Based on the new information gained from site reconnaissance, update the CSM as appropriate. Identify any remaining data gaps in the CSM so that these data needs can be incorporated into the Sampling and Analysis Plan (SAP).
2.2
Step 2: Comparing CSM to SSL Scenario
The Soil Screening Guidance for Radionuclides is likely to be appropriate for sites where residential land use is reasonably anticipated. However, the CSM may include other sources and exposure pathways that are not covered by this guidance. Compare the CSM with the assumptions and limitations inherent in the SSLs to determine whether additional or more detailed assessments are needed for any exposure pathways or radionuclides. Early identification of areas or conditions where SSLs are not applicable is important so that other characterization and response efforts can be considered when planning the sampling strategy.
2.2.1 Identify Pathways Present at the Site Addressed by Guidance. The following are
rationale for excluding this exposure pathway should be consistent with EPA ground water policy (U.S. EPA, 1988a, 1990a, 1992a, 1992c, and 1993b). In addition to the more common pathways of exposure in a residential setting, concerns have been raised regarding the potential for migration of radon from subsurface soils into basements. The dominant factor in indoor radon levels is home construction practices and the extent to which these practices employ radonresistant techniques. Homes built atop soil with identical levels of radium can have orders of magnitude differences in indoor radon levels depending on the extent to which radon-resistant techniques are used. As NORM, radium is present in all soils. Reducing the radium content in the soil may not result in any reduction in indoor radon levels. However, taking simple and inexpensive steps in home construction will ensure that radon levels in homes are kept below ARAR levels. For existing homes with elevated levels of radon, a variety of methods can be used to reduce radon concentrations to ARAR levels. Discussion of radon mitigation standards may be found in several EPA publications, including Radon Mitigation Standards, EPA 402-R-93-078. Also note that potential soil Applicable or Relevant and Appropriate Requirements (ARARs) exist for radon under 192.12(b)(1) and 192.41(b). For further guidance on using these ARARs, see the August 1997 memorandum from Stephen Luftig (OERR) and Larry Weinstock (ORIA) titled “Establishment of cleanup levels for CERCLA sites with radioactive contamination,” OSWER Directive 9200.418, (U.S. EPA, 1997c).
+ There are potential terrestrial or aquatic ecological concerns. + There are other likely human exposure pathways that were not considered in development of the SSLs (e.g., local fish consumption; raising of beef, dairy, or other livestock; recreational activities such as playground activities, hunting and fishing, construction activities). + There are unusual site conditions such as large areas of contamination, unusually high fugitive dust levels due to soil being tilled for agricultural use, or heavy traffic on unpaved roads. + There are certain subsurface site conditions such as karst, fractured rock aquifers, or contamination extending below the water table, that result in the screening models not being sufficiently conservative. + There is the probability of prolonged skin contact with high levels of high energy beta-emitting contaminants for periods of time (several years), and all other pathways show a very low risk. The skin contact exposure pathway is normally several orders of magnitude lower than either the inhalation, ingestion, or external exposure pathway (depending on the radionuclide, see Section 2.2.1) due to very low risk coefficients and normal hygiene practices (washing skin routinely).
2.2.3 Compare Available Data to Background. EPA may be concerned with two types
of radioactivity background at sites: naturally-occurring and anthropogenic. Naturally-occurring background radiation is much more ubiquitous in the environment than naturally-occurring background chemicals. Natural background radiation includes terrestrial radionuclides, cosmic radiation and cosmogenic radionuclides. Anthropogenic background consists of manmade isotopes which are distributed in the environment due primarily to releases from nuclear weapons testing and to the very small, but measurable releases from nuclear facilities. A comparison of available data (e.g., State soil surveys or other sources of soil radioactivity analyses) on local background concentrations with generic SSLs may indicate whether background concentrations at the site are elevated. Generally, EPA does not cleanup below natural background levels; however, where
2-4
2.2.2 Identify Additional Pathways Present at the Site Not Addressed by Guidance. The presence of additional pathways does
not preclude the use of SSLs in site areas that are currently residential or likely to be residential in the future. However, the risks associated with these additional pathways should also be considered in the RI/FS to determine whether SSLs are adequately protective. Where the following conditions exist, a more detailed site-specific study should be performed: + The site is adjacent to bodies of surface water where the potential for contamination of surface water by overland flow or release of contaminated ground water into surface water through seeps should be considered.
anthropogenic background levels exceed SSLs and EPA has determined that a response action is necessary and feasible, EPA's goal will be to develop a comprehensive response to address area soils. This will often require coordination with different authorities that have jurisdiction over other sources of contamination in the area. This will help avoid response actions that create “clean islands” amid widespread contamination. Knowledge of background radionuclide concentrations at the site is critical when screening site soils, since facility operations may have contaminated site soils with some of the same radionuclides that are found naturallyoccurring in background soil. In many cases, the concentration of the radionuclide of concern in background soil, and the variability of the background soil concentration, may be much greater than the screening level. In these situations, the site manager should not exclude the radionuclide of potential concern from being evaluated in the risk assessment, as the contamination from the facility may pose a threat to human health and the environment. Risk management options for the radionuclides of concern will be evaluated in the CERCLA remedy selection process. Note that potential soil ARARs exist for several of the more common naturally-occurring radionuclides (226Ra, 228 Ra, 230Th, 232Th, 235U, and 238U) under 40 CFR Part 192.12(a), Part 192.32(b)(2), and Part 192.41, and 10 CFR Part 40 Appendix A, I, Criterion 6(6). For further guidance on using these ARARs, see OSWER Directive 9200.4-25 (U.S. EPA, 1998b), dated February 12, 1998 and OSWER Directive 9200.4-35P (U.S. EPA, 2000a), dated April 11, 2000.
+ Surface soil + Subsurface soil + Soil characteristics To develop sampling strategies that will properly assess site contamination, EPA recommends that site managers consult with the technical experts in their Region, including risk assessors, toxicologists, health physicists, chemists and hydrogeologists. These experts can assist the site manager to use the Data Quality Objectives (DQO) process to satisfy Superfund program objectives. The DQO process is a systematic planning process developed by EPA to ensure that sufficient data are collected to support EPA decision making. Using the DQO Process ensures that the type, quantity, and quality of environmental data used in decision making will be appropriate for the intended medium. A full discussion of the DQO process is provided in Data Quality Objectives for Superfund: Interim Final Guidance (U.S. EPA, 1993a) and the Guidance for the Data Quality Objectives Process (U.S. EPA, 1994b). In addition, MARSSIM provides extensive discussions of the DQO Process as it is applied to conducting radiation site surveys. Most key elements of the DQO process have already been incorporated as part of this Soil Screening Guidance for Radionuclides. Exhibit 5 shows the general components of the DQO process as it is applied to environmental data analysis. Detailed DQOs for the soil screening process are provided in Attachment B. Exhibit 6 expands upon step 4 of the DQO process, and provides additional guidance to define site study boundaries The remaining elements involve identifying the site-specific information needed to calculate SSLs. The following sections present an overview of the sampling strategies needed to use the Soil Screening Guidance for Radionuclides. For a more detailed discussion, see the supporting Soil Screening Guidance for Radionuclides: Technical Background Document (TBD).
2.3
Step 3: Defining Data Collection Needs for Soils
Once the CSM has been developed and the site manager has determined that the Soil Screening Guidance for Radionuclides is appropriate to use at a site, an Sampling and Analysis Plan (SAP) should be developed. Attachment A, the Conceptual Site Model Summary, lists the data needed to apply the Soil Screening Guidance for Radionuclides. The summary will help identify data gaps in the CSM that require collection of site-specific data. The soil SAP is likely to contain different sampling strategies that address:
2-5
2.3.1 Stratify the Site Based on Existing Data. At this point in the soil screening process,
existing data can be used to stratify the site into three types of areas requiring different levels of investigation: + Areas unlikely to be contaminated
+ +
Areas known to be highly contaminated Areas that may be contaminated and cannot be ruled out.
This guidance suggests an upper bound for the size of an EA is 2,000 m2 (0.5-acre). This limitation on EA size is intended to ensure that each area is assigned an adequate number of data points. Because the number of samples is independent of the EA size, limiting the size of an EA ensures that the default sample density does not exceed 333 m2 per sample. This also serves to limit the sample spacing. The statistical basis for the default sample number is provided in Section 3.3.3.
Areas that are unlikely to be contaminated generally will not require further investigation if historical site use information or other site data, which are reasonably complete and accurate, confirm this assumption. These may be areas of the site that were completely undisturbed by activities at the facility. A crude estimate of the degree of soil contamination can be made for other areas of the site by comparing site concentrations to the generic SSLs in Appendix A of the TBD. Generic SSLs have been calculated for 60 radionuclides using default values in the SSL equations, resulting in conservative values that will be protective for the majority of site conditions. The pathway-specific generic SSLs can be compared with available concentration data from previous site investigations or removal actions to help divide the site into areas with similar levels of soil contamination and develop appropriate sampling strategies. The surface soil sampling strategy discussed in this document is most appropriate for those areas that may be contaminated and can not be designated as uncontaminated. Areas which are known to be contaminated (based on existing data) will be investigated and characterized in the RI/FS.
2.3.3 Develop Sampling and Analysis Plan for Surface Soil. The surface soil sampling
strategy is designed to collect the data needed to evaluate exposures via direct ingestion of soil, inhalation of fugitive dusts, external radiation exposure, ingestion of homegrown produce pathways, as well as migration of contaminants to groundwater. The SAP developed for surface soils should specify sampling and analytical procedures as well as the development of QA/QC procedures. To identify the appropriate analytical procedures, the screening levels must be known. If data are not available to calculate site-specific SSLs (Section 2.5.1), then the generic SSLs in Appendix A of the TBD should be used. The depth over which surface soils are sampled should reflect the CSM and the pathway assumptions that form the basis for the SSL determination. The residential setting used to develop the SSLs for each pathway assumes that: 1) there is no clean cover of soil; 2) the top few centimeters of soil are available for resuspension in air; 3) the top 15 cm of contaminated soil are homogenized by agricultural activities (e.g., plowing); 4) there is a sufficiently large area and depth of contamination to approximate an infinite slab source for external exposure purposes; 5) there is enough land for the residential garden to supply one-half of the residents’ annual produce consumption; and, 6) while the plant root system grows to a depth of 1 meter, most plant nutrients are obtained from within the upper 20 cm of soil. Further discussion of the basis for these assumptions is provided in the appropriate pathway discussions in Section 2.5.1. Note that the size, shape, and orientation of sampling volume (i.e., “support”) for heterogenous media have a significant effect on reported measurement values.
2-6
2.3.2
Identify Exposure Areas. An exposure
area (EA) is a physical area of a specified size and shape for which a separate decision will be made as to whether or not the area exceeds the screening criteria. To facilitate survey design and ensure that the number of survey data points for a specific site are relatively uniformly distributed among areas of similar contamination potential, the site is divided into EAs that share a common history or other characteristics, or are naturally distinguishable from other portions of the site (see Exhibit 6). An EA should not include areas that have potentially different levels of contamination. The EA’s characteristics should be generally consistent with the SSL exposure pathway modeling. EAs should be limited in size based on classification, exposure pathway modeling assumptions, and site-specific conditions.
Sample characteristics such as sample depth, volume, area, moisture level, and composition, as well as sample preparation techniques which may alter the sample, are important planning considerations for Data Quality Objectives. Comparison of data from methods that are based on different supports can be difficult. Defining the sampling support is important in the early stages of site characterization. This may be accomplished through the DQO process with existing knowledge of the site, contamination, and identification of the exposure pathways that need to be characterized. Refer to Preparation of Soil Sampling Protocols: Sampling Techniques and Strategies (U.S. EPA, 1992e) for more information about soil sampling support. As explained in the Supplemental Guidance to RAGS: Calculating the Concentration Term (U.S. EPA, 1992d), an individual is assumed to move randomly across an exposure area (EA) over time, spending equivalent amounts of time in each location. Thus, the concentration contacted over time is best represented by the spatially averaged concentration over the EA. Ideally, the surface soil sampling strategy would determine the true population mean of radionuclide soil concentrations in an EA. Because determination of the “true” mean would require extensive sampling at high costs, the maximum radionuclide concentration from composite samples is used as a conservative estimate of the mean. The number of samples required to satisfy the DQOs for the survey is then based on the selection of a statistical test, which in turn is based on whether or not the radionuclide of concern is present in background. For guidance when the radionuclide of concern is present in background, refer to the TBD. Radionuclide Not Present in Background. For those radionuclides that are not generally present in background, measurement of background soil concentration is not necessary and radionuclide concentrations are compared directly with the screening level. With only a single set of EA samples, the statistical test used here is called a one-sample test. The one-sample test may also be used for those radionuclides that are present in background but are found only at a small fraction of the SSL. In this case, the background contribution is included in the radioactivity in the samples for the EA. Thus, the total concentration is compared to the screening level. This option should
2-7
only be used if one expects that ignoring the background concentration will not affect the outcome of the statistical test. The advantage of ignoring a small background contribution is that a background reference area is not required and no background sampling is needed. This may simplify the soil screening process considerably.
Exhibit 5: Data Quality Objectives Process
1. S tate the Pro blem
S um m ariz e th e c onta mina tio n pr oble m th at w ill req uir e ne w en v iro nm e ntal data, and ide ntify th e re sour ces av ai labl e to res olv e the pro blem .
2. Id en tif y the Deci sion
Iden tify the de cisi on that req uire s ne w en v ir onm e ntal data to a ddr ess th e c on tam ina tio n p rob lem .
3. Ide ntify Inputs to the Decis ion
Iden tify the inform ati on nee ded to su ppo rt the de cis ion , a nd speci fy w h ich inputs req uire new env i ronm e ntal m eas ureme nts.
4. De fine t he S t udy Bo und ari es
S pec ify the spa tia l an d tem po ral asp ects of th e en v ir onm e ntal m ed ia tha t the data m ust re pre sen t to s upp ort th e d eci sio n.
Ex pa nde d in Ex hibit 6
5. Deve lop a D ecis ion Rule
Dev e lop a lo gica l " i f .. . th en . .." statemen t th at d efin es the c ondi tion s th at w oul d ca use the deci sion mak er to ch oose amo ng a lter nati v e a ctio ns.
6. S pec ify Limi ts on D ecis ion Erro rs
S pecify the deci sion maker 's ac cepta ble li m its on de cisio n err ors, w hich are used to establ ish per for man ce g oal s for l imi tin g un cer tain ty in the data.
7. O p timize the D esign for O btaini ng Dat a
Identify the most res ourc e-effectiv e samp ling and ana ly sis des ign for a data th at a re ex pected to sa tisfy the DQ O s.
Sur fa c e Soils Ex pa nde d in Ex hibit 7
Subs ur fa c e Soils Ex pa nde d in Ex hibit 8
2-8
Exhibit 6: Defining the Study Boundaries
Study Boundaries
1.
Define Geographic Area of the Investigation
Surface Soil (usually top 15 centimeters)
2.
Define Population of Interest
Subsurface Soil Water Table (Saturated Zone)
3.
Stratify the Site
Area Unlikely to be Contaminated
Area of Suspected Contamination
Area of Known Contamination (possible source)
4.
Define Scale of Decision Making for Surface or Subsurface Soils SURFACE SOILS
0.5-acre exposure areas (EAs)
SUBSURFACE SOILS
Contaminant Source
2-9
The Max test, which is used when the radionuclide of concern is not generally present in background, is a simple decision rule comparing the maximum radionuclide concentrations of composite samples with soil screening levels. Another, more complex strategy called the Sign test is presented in Part 6 of the TBD. The User’s Guide for Radionuclides uses the Max test rather than the Sign test to maintain consistency with the approach taken in the SSG for chemicals. While the Sign test is a more complex statistical method than the Max test, it is based on the same statistical null hypothesis (i.e., the EA requires further investigation). Some EAs that cannot be screened out with the Max test could be screened out with the Sign test since it uses a less conservative estimate of the mean concentration than does the Max test. In addition to determining the mean concentration of a radionuclide in an EA, it is important to identify the presence of small areas of elevated activity. This is done by the performance of scanning surveys. The sensitivity of scanning surveys will be insufficient to detect small areas of elevated activity for most radionuclides with levels of contamination as low as those of the SSLs calculated for large areas of uniform contamination. However, standard scanning survey techniques may be able to detect SSLs calculated for smaller areas of contamination. Scan surveys are intended to provide a degree of confidence that any significant areas of elevated activity are identified. Therefore, scanning surveys should be performed for all EAs prior to sampling. The extent of the survey coverage should be dictated by the potential for small areas of elevated activity in the EA. EAs with a high potential for small areas of elevated activity should receive 100% coverage. In EAs with a very low potential for small areas of elevated activity, scanning surveys should be performed in at least 10% of the area. In such cases, the areas selected for scan should be those with highest potential based on professional judgement. Due to the limited sensitivity of scan surveys, any small areas of elevated activity found during the survey should be identified for further investigation (i.e., not screened out). Exhibit 7 provides a summary of SAP design considerations for EAs when the radionuclides of concern for surface soils are not present in background. The following strategy can be used for surface soils to estimate the mean concentration of radionuclides in an
EA when the radionuclide of concern is not present in background. + Divide areas to be sampled in the screening process into 0.5-acre exposure areas, the size of a suburban residential lot. If the site is currently residential, the exposure area should be the actual residential lot size. The exposure areas should not be laid out in such a way that they unnecessarily combine areas of high and low levels of contamination. The orientation and exact location of the EA, relative to the distribution of the contaminant in the soil, can lead to instances where sampling the EA may have contaminant concentration results above the mean, and in other instances, results below the mean. Composite surface soil samples. Because the objective of surface soil screening is to estimate the mean contaminant concentration, the physical “averaging” that occurs during compositing is consistent with the intended use of the data. Compositing allows sampling of a larger number of locations while controlling analytical costs, since several individual samples are physically mixed (homogenized) and one or more subsamples are drawn from the mixture and submitted for analysis. Strive to achieve a Type I (false negative) error rate of 5 percent (i.e., in only 5 percent of the cases, soil contamination is assumed to be below the screening level when it is really above the screening level). EPA also strives to achieve a 20 percent Type II (false positive) error rate (i.e., in only 20 percent of the cases, soil contamination is assumed to be above the screening level when it is really below the screening level). These error rate goals influence the number of samples to be collected in each exposure area. For this guidance, EPA has defined the “gray region” as one-half to 2 times the SSL. Thus, the width of the gray region, also known as the shift, ∆, is equal to 1.5 times the SSL. Refer to Section 2.6 for further discussion. The default sample size chosen for this guidance (see Exhibit 7) provides adequate coverage for a coefficient of variation (CV) based upon 250 percent variability in contaminant values (CV=2.5). (If a CV larger than 2.5 is expected, use an appropriate sample size from the table in Exhibit 7 of the User’s Guide, or tables in the TBD.)
+
+
+
2-10
+
Take six composite samples for each exposure area with each composite sample made up of four individual samples. Exhibit 7 shows other sample sizes needed to achieve the decision error rates for other CVs. Collect the composites randomly across the EA and through the top 15 centimeters of soil, which are of greatest concern for the external exposure and consumption of homegrown produce pathways. Analyze the six samples per exposure area to determine the radionuclides present and their concentrations.
The SAP developed for subsurface soils should specify sampling and analytical procedures as well as the development of QA/QC procedures. To identify the appropriate procedures, the SSLs must be known. If data are not available to calculate site-specific SSLs (Section 2.5.2), the generic SSLs in Appendix A of the TBD should be used. The primary goal of the subsurface sampling strategy is to estimate the mean radionuclide concentration and average soil characteristics within the source area. As with the surface soil sampling strategy, the subsurface soil sampling strategy follows the DQO process (see Exhibits 5, 6, and 8). Exhibit 8 provides a summary of SAP design considerations for subsurface soils. If the radionuclide of concern is not present in background, the decision rule is based on comparing the mean radionuclide concentration within each contaminant source with source-specific SSLs. Current investigative techniques and statistical methods cannot accurately determine the mean concentration of subsurface soils within a contaminated source without a costly and intensive sampling program that is well beyond the level of effort generally appropriate for screening. Thus, conservative assumptions should be used to develop hypotheses on likely contaminant distributions.
+
For further information on compositing across or within EA sectors, developing a random sampling strategy, and determining sample sizes that control decision error rates, refer to the TBD. Note that the Max test requires a Data Quality Assessment (DQA) test following sampling and analysis (Section 2.4.2) to ensure that the DQOs (i.e., decision error rate goals) are achieved. If DQOs are not met, additional sampling may be required.
2.3.4 Develop Sampling and Analysis Plan for Subsurface Soils. The
subsurface and surface soil sampling strategies differ because the exposure mechanisms differ. Exposure to surface contaminants occurs as individuals move around a residential lot. The surface soil sampling strategy reflects this type of exposure. In general, exposure to subsurface contamination occurs when radionuclides migrate down to an underlying aquifer. Thus, subsurface sampling focuses on collecting the data required for modeling the migration to ground water pathway. Measurements of soil characteristics and estimates of the area and depth of contamination and the average contaminant concentration in each source area are needed to supply the data necessary to calculate the migration to ground water SSLs. Source areas are the decision units for subsurface soils. A source area is defined by the horizontal extent, and vertical extent or depth of contamination. Sites with multiple sources should develop separate SSLs for each source.
2-11
Exhibit 7: Designing a Sampling and Analysis Plan for Surface Soils Radionuclide Not Present in Background
1. Subdivide Site Into EAs
EA For surface soils, the individual unit for decision making is an “EA,” or exposure area. It measures 0.5 acre in area or less.
2. Divide EA Into a Grid
1
2
3
4
This step defines the number of specimens (N) that will make up one composite sample.
1
6 3 2 3 5 5 2 6 2 4 3 6 1 5
4
2
3. Organize Surface Sampling Program for EA
1
4
1
5
a. Placement of sample locations on the grid was developed using a default sample size of 6 (which is based on acceptable error rates for a CV of 2.5) and a stratified random sampling pattern. b. Potential for small areas of elevated activity determines degree of scan coverage.
4 3 6
If the EA CV is suspected to be greater than 2.5, use the table below to select an adequate sample size or refer to the TBD for other sample design options.
Probability of Decision Error at 0.5 SSL and 2 SSL Using Max Test Sample Sizeb CV=2.5a E CV=3.0
d
CV=3.5 E
e
CV=4.0 E
0.5
c
E
2.0
E
0.5 E2.0
0.5 E2.0
0.11 0.09 0.07 0.07
0.5 E2.0
0.16 0.15 0.09 0.08
6 7 8 9
C = 4 specimens per composite 0.21 0.08 0.28 0.11 0.25 0.25 0.28 0.05 0.04 0.03 0.31 0.36 0.36 0.08 0.05 0.04
0.31 0.36 0.42 0.44
0.35 0.41 0.41 0.48
a The CV is the coefficient of variation for individual, uncomposited measurements across the entire EA, including measurement error. b Sample size (N) = number of composite samples c E0.5 = Probability of requiring further investigation when the EA mean is 0.5 SSL d E2.0 = Probability of not requiring further investigation when the EA mean is 2.0 SSL e C = number of specimens per composite sample, when each composite consists of points from a stratified random or systemic grid sample from across the entire EA. NOTE: All decision error rates are based on 1,000 simulations that assume that each composite is representative of the entire EA, half the EA has concentrations below the limit of detection, and half the EA has concentrations that follow a gamma distribution (a conservative distributional assumption).
2-12
This guidance bases the decision to investigate a source area further on the highest mean soil boring contaminant concentration within the source, reflecting the conservative assumption that the highest mean subsurface soil boring concentration among a set of borings taken from the source area represents the mean of the entire source area. Similarly, estimates of contaminant depths should be conservative. The investigation should include the maximum depth of contamination encountered within the source without going below the water table. For each source, the guidance recommends taking 2 or 3 soil borings located in the areas suspected of having the highest contaminant concentrations within the source. These subsurface soil sampling locations are based primarily on knowledge of likely surface soil contamination patterns (see Exhibit 6) and subsurface conditions. However, buried sources may not be discernible at the surface. Information on past practices at the site included in the CSM can help identify subsurface source areas. Take soil cores from the soil boring using either split spoon sampling or other appropriate sampling methods. Description and Sampling of Contaminated Soils: A Field Pocket Guide (U.S. EPA, 1991f), and Subsurface Characterization and Monitoring Techniques: A Desk Reference Guide, Vol. I & II (U.S. EPA, 1993e), can be consulted for information on appropriate subsurface sampling methods. For radioactive contaminants, core samples may also be obtained and monitored intact in the field to determine if layers of radioactivity are present. In addition, the use of a subsurface sampling technique, which results in a borehole or soil face, may be “logged” using a gamma scintillation detector. This enables scanning of the exposed soil surface to identify radioactive contamination within small fractions of hole depth, thus facilitating the identification of the presence and depth distribution of subsurface radioactivity. This information may be used to direct further core sampling and laboratory analysis as warranted. Sampling should begin at the ground surface and continue until either no contamination is encountered or the water table is reached. Subsurface sampling intervals can be adjusted at a site to accommodate site-specific information on subsurface contaminant distributions and geological conditions (e.g., very deep water table, very thick uncontaminated unsaturated
zone, user well far beyond edge of site, soils underlain by karst or fractured rock aquifers). Sample splits and subsampling should be performed according to Preparation of Soil Sampling Protocols: Sampling Techniques and Strategies (U.S. EPA, 1992e). If each subsurface soil core segment represents the same subsurface soil interval (e.g., 2 feet), the average concentration from the surface to the depth of contamination is the simple arithmetic average of contaminant concentrations measured for core samples representative of each of the 2-foot segments from the surface to the depth of contamination. However, if the sample intervals are not all of the same length (e.g., some are 2 feet while others are 1 foot or 6 inches), the calculation of the average concentration in the total core must account for the different lengths of the segments. If ci is the concentration measure in a core sample, representative of a core interval or segment of length li, and the n-th segment is considered to be the last segment sampled in the core (i.e., the n-th segment is at the depth of contamination), the average concentration in the core from the surface to the depth of contamination should be calculated as the following depth-weighted average ( ). c Alternatively, the average boring concentration can be determined by adding the total contaminant activities together (from the sample results) for all sample segments to get the total contaminant activity for the boring. The total contaminant activity is then divided by the total dry weight of the core (as determined by the dry bulk density measurements) to estimate average soil boring concentration. Finally, the soil investigation for the migration to ground water pathway should not be conducted independently of ground water investigations. Contaminated ground water may indicate the presence of a nearby source area that would leach contaminants from soil into aquifer systems. 2.3. 5 Develop Sampling and Analysis Plan to Determine Soil Characteristics. The soil parameters necessary for SSL calculations are soil texture, dry bulk density, and pH. Although laboratory measurements of these parameters cannot be obtained under Superfund’s Contract Laboratory Program (CLP), independent soil testing laboratories across the country can perform these tests at a relatively low cost.
2-13
To appropriately apply the migration-to-ground water models, average or typical soil properties should be used for a source in the SSL equations (see Step 5). Take samples for measuring soil parameters with samples for measuring contaminant concentrations. If possible, consider splitting single samples for contaminant and soil parameter measurements. A number of soil testing laboratories can handle and test radioactive samples. However, if testing contaminated samples for soil parameters is a problem, samples may be obtained from clean areas of the site as long as they represent the same soil texture and are taken from approximately the same depth as the contaminant concentration samples. Soil Texture. Soil texture class (e.g., loam, sand, silt loam) is necessary to estimate average soil moisture conditions and to apply the Hydrological Evaluation of Landfill Performance (HELP) model to estimate infiltration rates (see Attachment A). The appropriate texture classification is determined by a particle size analysis and the U.S. Department of Agriculture (USDA) soil textural triangle shown in Exhibit 9. This classification system is based on the USDA soil particle size classification. The particle size analysis method in Gee and Bauder (1986) can provide this particle size distribution. Other methods are appropriate as long as they provide the same particle size breakpoints for sand/silt (0.05 mm) and silt/clay (0.002 mm). Field methods are an alternative for determining soil textural class. Exhibit 9 presents an example from Brady (1990).
pH. Soil pH is used to select site-specific partition coefficients. This simple measurement is made with a pH meter in a soil/water slurry (McLean, 1982) and may be measured in the field using a portable pH meter.
2.3.6 Determine Analytical Methods and Establish QA/QC Protocols. Assemble a list of
feasible sampling and analytical survey methods during this step. Routinely, radiological soil surveys are conducted using a mix of three types of radiation measurement methods: 1) scans, 2) direct measurements, and 3) sampling and laboratory analysis. Based on the potential radionuclide contaminants and their associated radiations, the detection sensitivities of various instruments and techniques are determined and documented. Methods must not only be chosen based on their reliability and suitability to the physical and environmental conditions at the site, but they must be capable of detecting the radionuclides of concern to the appropriate minimum detectable concentration (MDC). During survey design, it is generally considered good practice to select a measurement system with an MDC between 10-50% of the SSL. For soil screening purposes, most SSLs for radionuclides are too low to be detected using scans and direct measurements. Therefore, sampling and laboratory analysis must be the primary means of soil screening for the majority of radionuclides. Once the survey design and sampling methods are selected, appropriate standard operating procedures (SOPs) should be developed and documented. Both sample depth and area are considerations in determining appropriate sample volume, and sample volume is a key consideration for determining the laboratory MDC. The depth should also correlate with the CSM developed for the site. Field methods will be useful in defining the study boundaries (i.e., area and depth of contamination) during both site reconnaissance and sampling. The design and capabilities of field portable instrumentation are rapidly evolving. Documents describing the standard operating procedures for field instruments are available though the National Technical Information Service (NTIS). Additionally, MARSSIM provides further information on field (Chapter 6) and laboratory (Chapter 7)
to calculate total soil porosity and can be determined for any soil horizon by weighing a thin-walled tube soil sample (e.g., Shelby tube) of known volume and subtracting the tube weight [American Society for Testing and Materials (ASTM) D 2937]. Determine moisture content (ASTM 2216) on a subsample of the tube sample to adjust field bulk density to dry bulk density. The other methods (e.g., ASTM D 1556, D 2167, D 2922) are generally applicable only to surface soil horizons and are not appropriate for subsurface characterization. ASTM soil testing methods are readily available in the Annual Book of ASTM Standards, Volume 4.08, Soil and Rock; Building Stones, available from ASTM, 100 Barr Harbor Drive, West Conshohocken, PA, 19428.
Dry Bulk Density. Dry soil bulk density (ρ b) is used
2-14
measurement methods and instrumentation for radionuclides. Appendix H of MARSSIM describes typical field and laboratory equipment plus associated cost and instrument sensitivities. MARSSIM also discusses the concept of detection sensitivity and provides guidance on determining sensitivities and selecting appropriate measurement methods. SAP quality control may be thought of in three parts: 1) determining the type of QC samples needed to detect precision or bias; 2) determining the number of samples as part of the survey design; and 3) scheduling sample collections throughout the survey process to identify and control sources of error and uncertainties. Because a great amount of variability and bias can exist in the collection, subsampling, and analysis of soil samples, some effort should be made to characterize this variability and bias. A Rationale for the Assessment of Errors in the Sampling of Soils (U.S. EPA, 1990c) outlines an approach that advocates the use of a suite of QA/QC samples to assess variability and bias. Field duplicates and splits are some of the best indicators of overall variability in the sampling and analytical processes. At least 10 percent of both the discrete samples and the composites should be split and sent to a laboratory for confirmatory analysis. (Quality Assurance for Superfund Environmental Data Collection Activities, U.S. EPA, 1993c). Regardless of whether surface or subsurface soils are sampled, the Superfund quality assurance program guidance (U.S. EPA, 1993c) should be consulted. In addition, Specifications and Guidelines for Quality Systems for Environmental Data Collection and Environmental Technology Program (ANSI/ASCQ, 1994) describes a basic set of specifications and guidelines by which a quality system for programs involving environmental data collection and environmental technology can be planned, implemented, and assessed. Standard limits on the precision and bias of sampling and analytical operations conducted during sampling do apply and should be followed to give consistent and defensible results.
to the analytical laboratory and field methods specified in the SAP. Results of the analyses should identify the concentrations of potential radionuclides of concern for which site-specific SSLs will be calculated.
2.4.1
Delineate Area and Depth of Source.
Both spatial area and depth data, as well as soil characteristic data, are needed to calculate site-specific SSLs for the external exposure and migration to ground water pathways. Site information from the CSM or prior surveys can be used to estimate the areal extent of the sources.
2.4.2 Perform DQA Using Sample Results. Data Quality Assessment (DQA) is a
scientific and statistical evaluation that determines if the data are of the right type, quality, and quantity to support their intended use. The nature of the DQA is dependent upon whether the radionuclide of concern is present in background. For guidance for performing DQA when the radionuclide of concern is present in background, refer to the TBD. The following is a discussion of DQA for radionuclides not present in background. Radionuclide Not Present in Background. After sampling has been completed, a DQA should be conducted if all composite samples are less than 2 times the SSL. This is necessary to determine if the original CV estimate (2.5), and hence the number of samples collected (6), was adequate for screening surface soils.
2.4 Step 4: Sampling and Analyzing Site Soils & DQA
Once the sampling strategies have been developed and implemented, the samples should be analyzed according
2-15
Exhibit 8: Designing a Sampling and Analysis Plan for Subsurface Soils Radionuclide Not Present in Background
1.
De lin e ate So urce Are a
Co nta m i na n t S o u rce
Soil Borings
2.
Cho o se Su bsu rface So il Samp lin g Lo catio n s De sig n Sub su rface Samp lin g an d An alysis Plan
La b/Fie ld Ana lys is for s oil pa ra me te rs b S oil Boring
(dept h be low grou nd s ur fa ce i n fe et)
a
Fo r scre en i n g p u rp o se s, E PA re com m e n d s d ri l li n g 2 to 3 b o ri n gs p e r so u rce a re a a n d a n eq u i va le n t n umb e r i n a b a ckgro u n d re fe re n ce a r a e (w he n ra di o n u cli d e i s p re sen t i n b ackg ro u n d) i n a re as o f h i g h e st su sp e cte d co n ce n tra ti o n s. S o i l sam p l i n g sh o u ld n o t e xten d p a st wa te r ta b l e o r sa tura te d zon e .
3.
La b Ana lys is for s oil c onta m inants in s ourc e a re a a nd ba c k ground re fe renc e a re a ( he n w ra dionuc lide is pre s ent in ba c k ground)
Sam ple 1 2 Sam ple 2 4 Sam ple 3 6 Sam ple 4 8 8 6 4 2
Sam ple 1
Sam ple 2
Sam ple 3
Sam ple 4
Sam ple 5 10
a
Sam ple 5 10
Pic ture depic ts a c ontinuous boring with 2 f oot s egm ents . F or inf orm ati n on other m ethods s uc h as interv al sam pling and o depth weighted analy s s , pleas e ref er to 2. 3.3 of the U s er's Guide or 4.2 of the T . i BD Soil Tex t ure, D ry Bulk D ens ity , Soil Organic C arbon M ois ture Cont ent, pH . R etain sam ples f or pos s ibl dis c rete c ontam n ant e i s am pli g. n
b
2-16
Exhibit 9: U.S. Department of Agriculture soil texture classification
100 10
90
80
20
70
30 Clay 40
Percent Clay
60
Percent Silt
50 Clay
50 Sandy Clay Clay Loam 30 Sandy Clay Loam
40
60 Silty Clay Loam 70
20 Sandy Loam 10 Sand Loamy Sand
80 Loam Silt Loam 90 Silt 100
100
90
80
70
60
50 40 Percent Sand
30
20
10
Criteria Used with the Field Method for Determining Soil Texture Classes Criterion Individual grains visible to eye 2. Stability of dry clods 3. Stability of wet clods 4. Stability of "ribbon" when wet soil rubbed between thumb and fingers 1. Sand Yes Do not form Unstable Does not form Sandy loam Yes Do not form Slightly stable Does not form Loam Some Silt loam Few Clay loam No Hard and stable Very stable
(Source: Brady, 1990) Clay No Very hard and stable Very stable Very long, flexible
Easily Moderately broken easily broken Moderately Stable stable Does not formBroken appearance
Thin, will break
Particle Size, mm 0.002 U.S. Department of Agriculture Clay Silt Sand Source: USDA. 0.05 Very Fine 0.10 0.25 0.5 1.0 2.0 Very Coarse Gravel
Fine Med. Coarse
2-17
To conduct the DQA for a composite sample whose mean is below 2 SSL, first calculate the sample CV for
the EA in question from the sample mean ( ), the x number of specimens per composite sample (C), and sample standard deviation(s) as follows:
CV
C s . x
Use the sample size table in Exhibit 7 to check, for this CV, whether the sample size is adequate to meet the DQOs for the sampling effort. If sampling DQOs are not met, supplementary sampling may be needed to achieve DQOs. However, for EAs with small sample means (e.g., all composites are less than the SSL), the sample CV calculated using the equation above may not be a reliable estimate of the population CV (i.e., as x approaches zero, the sample CV will approach infinity). To protect against unnecessary additional sampling in such cases, compare all composites against the formula SSL÷ C . If the maximum composite sample concentration is below the value given by the equation, then the sample size may be assumed to be adequate and no further DQA is necessary. In other words, EPA believes that the default sample size will adequately support walk-away decisions when all composites are well below the SSL. The TBD describes the development of this formula and provides additional information on implementing the DQA process.
In the SSG for chemicals, SSLs are expressed in mass units of mg/kg (i.e., mg of chemical per kg of soil). The concentrations of radioactive material in soil could also be expressed in units of mass. Instead, they are expressed in the traditional radiological units of pCi/g (i.e., picograms of activity per gram of soil). These units reflect the number of atoms of the isotope undergoing radioactive transformation (referred to as radioactive decay) per unit time. For more information concerning activity and mass, refer to appendix B of the TBD. All SSL equations were developed to be consistent with RME in the residential setting. The Superfund program estimates the RME for chronic exposures on a sitespecific basis by combining an average exposure-point concentration with reasonably conservative values for intake and duration (U.S. EPA, 1989a; RAGS HHEM, Supplemental Guidance: Standard Default Exposure Factors, U.S. EPA, 1991a, Exposure Factors Handbook, U.S. EPA, 1997a). Thus, all site-specific parameters (soil, aquifer, and meteorologic parameters) used to calculate SSLs should reflect average or typical site conditions in order to calculate average exposure concentrations at the site. Equations for calculating SSLs are presented for surface and subsurface soils in the following sections. For each equation, site-specific input parameters are highlighted in bold and default values are provided for use when site-specific data are not available. Although these defaults are not worst case, they are conservative. At most sites, higher, but still protective SSLs can be calculated using site-specific data. The TBD describes development of these default values and presents generic SSLs calculated using the default values. Attachment D provides toxicity criteria for 60 radionuclides commonly found at NPL sites. These criteria were obtained from the Health Effects Assessment Summary Tables (HEAST), which is regularly updated. Prior to calculating SSLs at a site, check all relevant - radionuclide-specific values in Attachment D against values from HEAST at the following internet webpage http://www.epa.gov/superfund/programs/risk/calctool. htm. Only the most current values should be used to calculate SSLs.
2.4.3 Revise the CSM. Because these analyses reveal new information about the site, update the CSM accordingly. This revision could include identification of site areas that exceed the generic SSLs.
2.5
Step 5: C a l c u l a t i n g S i t e specific SSLs
With the soil properties data collected in Step 4 of the screening process, site-specific soil screening levels can now be calculated using the equations presented in this section. For a description of how these equations were developed, as well as background on their assumptions and limitations, consult the TBD.
2-18
Where toxicity values have been updated, the generic SSLs should also be recalculated with current toxicity information.
Equation 1:
Screening Level Equation for Ingestion of Radionuclides in Residential Soil TR SF × IR × 1x10 3 × EF × ED
2.5.1
SSL
Equations--Surface
Soils.
SSLsoil ing
Exposure pathways addressed in the process for screening surface soils include direct ingestion of soil, inhalation of fugitive dusts, ingestion of contaminated ground water, external radiation exposure, and ingestion of homegrown produce. Direct Ingestion of Soil. The Soil Screening Guidance for Radionuclides addresses chronic exposure to radionuclides through direct ingestion of contaminated soil in a residential setting. A number of studies have shown that inadvertent ingestion of soil is common among children age 6 and younger (Calabrese et al., 1989; Davis et al., 1990: Van Wijnen et al., 1990). In some cases, children may ingest large amounts of soil (i.e., 3 to 5 grams) in a single event. This behavior, known as pica, may result in relatively high short-term exposures to radionuclides in soil. Default values are used for all input parameters in the direct ingestion equations. The amount of data required to derive site-specific values for these parameters (e.g., soil ingestion rates, chemical-specific bioavailability) makes their collection and use impracticable for screening. Therefore, site-specific data are not generally available for this exposure route. The generic ingestion SSLs presented in Appendix A of the TBD are recommended for all NPL sites. However, for radionuclides, both the magnitude and duration of exposure are important. Duration is critical because the toxicity criteria are based on “lifetime average daily dose.” Therefore, the total intake, whether it be over 5 years or 50 years, is averaged over a lifetime of 70 years. To be protective of exposures to radionuclides in the residential setting, Superfund focuses on exposures to individuals who may live in the same residence for a high-end period of time (e.g., 30 years) because exposure to soil is higher during childhood and decreases with age. Equation 1 uses a time-weighted average soil ingestion rate for children and adults. The derivation of this time-weighted average is presented in U.S. EPA, 1991c.
Parameter/Definition (units) TR/target cancer risk (unitless) SFs /soil ingestion slope factor (pCi)-1 IRs /soil ingestion rate (mg/d) 1x10-3/conversion factor (g/mg) EF/exposure frequency (d/yr) ED/exposure duration (yr)
Default 10-6 See Attachment D 120 (age-averaged) -350 30
Inhalation of Fugitive Dusts. Inhalation of fugitive dusts is a consideration in surface soils. Equation 2 is used to calculate fugitive dust SSLs for radionuclides. This equation requires calculation of a particulate emission factor (PEF, Equation 3) that relates the concentration of contaminant in soil to the concentration of dust particles in air. This PEF represents an annual average emission rate based on wind erosion that should be compared with chronic health criteria. It is not appropriate for evaluating the potential for more acute exposures. Both the emissions portion and the dispersion portion (Q/C) of the PEF equation have been updated since the first publication of RAGS HHEM, Part B, in 1991. As in Part B, the emissions part of the PEF equation is based on the “unlimited reservoir” model developed to estimate particulate emissions due to wind erosion (Cowherd et al., 1985). The box model in RAGS HHEM, Part B has been replaced with a Q/C term derived from the modeling exercise using the AREA-ST model incorporated into EPA’s Industrial Source Complex Model (ISC2) platform. The AREA-ST model was run with a full year of meteorological data for 29 U.S. locations selected to be representative of a range of meteorologic conditions across the nation (EQ, 1993). The results of these modeling runs are presented in Exhibit 10 for square area sources of 0.5 to 30 acres in size. When developing a site-specific PEF for the inhalation pathway, place the site into a climatic zone (see Attachment B). Then select a Q/C value from Exhibit 10 that best represents a site’s size and meteorological
2-19
conditions. Additional information on the update of the PEF equation is provided in the TBD. Cowherd et al. (1985) present methods for site-specific measurement of the parameters necessary to calculate a PEF. The default PEF for radionuclides presented in Equation 2 is the same as the one given in the SSG for chemicals. The default parameter values shown in Equation 3 have been chosen using the guidance of Cowherd et al. (1985), based upon the assumption of a family garden. The calculated PEF thus accounts for the increase in the fugitive dust concentration anticipated with an area of tilled soils.
Equation 2: Screening Level Equation for Inhalation of Radioactive Fugitive Dusts from Residential Soil
SSL dust TR 1 3 ) × 1x10 × EF × ED × [ETo (ETi × DFi)] SFi × IRi × ( PEF
Equation 3: Derivation of the Particulate Emission Factor
PEF Q/C ×
3600 0.036 × (1 V) × (Um/Ut)3 × F(x) Default 1.32x10+9 90.80 0.5 (50%) 4.69 11.32 0.194
Parameter/Definition (units) PEF/particulate emission factor (m3/kg) Q/C/inverse of mean conc. at center of a 0.5-acre-square source (g/m2-s per kg/m3) V/fraction of vegetative cover (unitless) Um /mean annual windspeed (m/s) Ut /equivalent threshold value of windspeed at 7 m (m/s) F(x)/function dependent on Um/Ut derived using Cowherd et al. (1985) (unitless)
Parameter/Definition (units) TR/target cancer risk (unitless) SFi /inhalation slope factor (pCi-1) IRi /inhalation rate (m3/d) PEF/particulate emission factor (m3/kg) 1x10+3/conversion factor (g/kg) EF/exposure frequency (d/yr) ED/exposure duration (yr) ETo/exposure time fraction, outdoor (unitless) ETi/exposure time fraction, indoor (unitless) DFi/dilution factor for indoor inhalation, (unitless)
Default 10-6 See Attachment D 20 1.32x10+9 (Equation 3) -350 30 0.073 0.683 0.4
2-20
Exhibit 10 . Q/C Values by Source Area, City, and Climatic Zone
0.5 Acre Zone I Seattle Salem Zone II Fresno Los Angeles San Francisco Zone III Las Vegas Phoenix Albuquerque Zone IV Boise Winnemucca Salt Lake City Casper Denver Zone V Bismark Minneapolis Lincoln Zone VI Little Rock Houston Atlanta Charleston Raleigh-Durham Zone VII Chicago Cleveland Huntington Harrisburg Zone VIII Portland Hartford Philadelphia Zone IX Miami 82.72 73.44 62.00 68.81 89.51 95.55 64.04 84.18 69.41 69.23 78.09 100.13 75.59 83.39 90.80 81.64 73.63 79.25 77.08 74.89 77.26 97.78 83.22 53.89 81.90 74.23 71.35 90.24 85.61
1 Acre 72.62 64.42 54.37 60.24 78.51 83.87 56.07 73.82 60.88 60.67 68.47 87.87 66.27 73.07 79.68 71.47 64.51 69.47 67.56 65.65 67.75 85.81 73.06 47.24 71.87 65.01 62.55 79.14 74.97
Q/C (g/m2-s per kg/m3) 2 Acre 5 Acre 64.38 57.09 48.16 53.30 69.55 74.38 49.59 65.40 53.94 53.72 60.66 77.91 58.68 64.71 70.64 63.22 57.10 61.53 59.83 58.13 60.01 76.08 64.78 41.83 63.72 57.52 55.40 70.14 66.33 55.66 49.33 41.57 45.93 60.03 64.32 42.72 56.47 46.57 46.35 52.37 67.34 50.64 55.82 61.03 54.47 49.23 53.11 51.62 50.17 51.78 65.75 55.99 36.10 55.07 49.57 47.83 60.59 57.17
10 Acre 50.09 44.37 37.36 41.24 53.95 57.90 38.35 50.77 41.87 41.65 47.08 60.59 45.52 50.16 54.90 48.89 44.19 47.74 46.37 45.08 46.51 59.16 50.38 32.43 49.56 44.49 43.00 54.50 51.33
30 Acre 42.86 37.94 31.90 35.15 46.03 49.56 32.68 43.37 35.75 35.55 40.20 51.80 38.87 42.79 46.92 41.65 37.64 40.76 39.54 38.48 39.64 50.60 43.08 27.67 42.40 37.88 36.73 46.59 43.74
2-21
External Exposure to Radionuclides in Soil. An individual residing on a contaminated site will be exposed to radiation emitted by radionuclides present in the soil. In modeling external exposure to contaminated soil, the RAGS/HHEM Part B model (U.S. EPA, 1991c) does not account for the following processes: • • • • radioactive decay and progeny (i.e., radioactive daughters) ingrowth; correction factors for the geometry of the contaminated soil; depletion of the contaminated soil horizon by environmental processes, such as leaching, erosion, or plant uptake; and corrections for shielding by clean cover material.
and associated shielding effects by the simple application of a gamma shielding factor and indoor occupancy time adjustment.
Equation 4: Screening Level Equation for External Exposure to Radionuclides in Soil
SSLEXT TR EF SF e × ( ) × ED × ACF × [ETo (ETi × GSF)] 365
Parameter/Definition (units) TR/target cancer risk (unitless) SFe /external exposure slope factor (g/pCi/yr) EF/exposure frequency (d/yr) ED/exposure duration (yr) ACF/area correction factor ETo/exposure time fraction, outdoor (unitless) ETi/exposure time fraction, indoor (unitless) GSF/gamma shielding factor
Default 10-6 See Attachment D 350 30 0.9 0.073 0.683 0.4
The RAGS/HHEM Part B model does not provide any corrections for radioactive decay. When ingrowth of progeny is expected to be of importance, the progeny are included at the outset of the SSL calculations. The RAGS/HHEM Part B model assumes that an individual is exposed to a source geometry that is effectively an infinite slab. The concept of an “infinite slab” means that the thickness of the contaminated zone and its aerial extent are so large that it behaves as if it were infinite in its physical dimensions. In practice, soil contaminated to a depth greater than about 15 cm and with an aerial extent greater than about 1,000 m2 will create a radiation field comparable to that of an infinite slab. To accommodate the fact that in most residential settings the assumption of an infinite slab source will result in overly conservative SSLs, an adjustment for source area is considered to be an important modification to the RAGS/HHEM Part B model. Thus, an area correction factor, ACF, has been added to the calculation of SSLs. No soil depletion processes are assumed to take place. Accordingly, the SSL model assumes that the contaminated zone is a constant, non-depleting source of radioactivity. This assumption provides an upper bound estimate of exposure to radionuclides in soil. For the purposes of this report, adjustments for clean cover are not needed since, in all cases, it is assumed that the contaminated soil extends to the surface. The SSL model provides adjustments for indoor occupancy
With the exception of the area correction factor, default values are used for all input parameters in Equation 4 to calculate external exposure SSLs. The amount of data required to derive site-specific values for these parameters makes their collection and use impracticable for calculation of simple site-specific SSLs. Therefore, site-specific data are generally not available for this exposure pathway. Alternative area correction factors, for use when site-specific data are available, are discussed in the TBD. The generic SSLs presented in Appendix A of the TBD are recommended for all sites. Ingestion of Homegrown Produce. Persons living on a contaminated site may ingest radioactive material by consumption of plants grown in a family garden. In this model, the fruits and vegetables primarily become contaminated by root uptake of radionuclides contained in the pore water of the soil in which the plants are growing. The following factors have been added/changed for exposure through this pathway for radionuclides as compared to chemicals: • soil-plant transfer factors used to estimate root uptake from soil assume that the roots are wholly exposed to contaminated soil;
2-22
• • •
air deposition, rain splash, and irrigation are not included; environmental equilibria assumed to exist for estimating concentrations of 3H and 14C in plants; and no more than 50% of produce is assumed to be homegrown (i.e., contaminated plant fraction < 0.5), with adjustment for small site areas (i.e., <2,000 m2).
The model accounts for that uptake with a simple soilto-plant transfer factor. These soil-to-plant transfer factors have been developed based upon the assumption that the entire plant root system is wholly exposed to contaminated soil. If the plant roots extend to a depth of 100 cm but the radionuclide contaminants are confined to the upper 15 cm, an initial assumption may be that only 15% of the root system is active in accumulating contaminants and that the reported soil-to-plant transfer factors should be reduced by a correction factor of 0.15. However, the equation for calculation of SSLs for this pathway does not apply any reduction to the soil-to-plant transfer factors. The basis for this assumption is as follows. Most plant root systems are in fact very active in the upper soil horizon, especially in the upper 15 cm of soil. This point is illustrated in a number of ways: 1) by illustrations of root morphology and growth habit, 2) positive physiological factors including the availability of water, oxygen and nutrients near the soil surface, 3) negative physiological or agronomic factors—including subsurface soil compaction, subsurface zones of acidity, perched water tables, hypoxia, etc., 4) interactions with soil microbes—with a special focus on mychorrizal fungi, and 5) split root studies. Thus, roots commonly proliferate in the upper layers of soil. If one assumes that a plant is actively growing, then ion uptake characteristics and lateral root growth strongly suggest that simply attributing 15% of root uptake activity to the upper 15 cm of the soil is not a sound approach. Environmental forces may influence root growth to one or more meters in depth, but more so for obtaining water than nutrients. In reality, the upper 15 cm of soil may include 50% or more of the root system—and thus 50% or more of the ion uptake (SC&A, 1994). The decision to not include air deposition or rain-splash does not affect any radionuclides because the increase in
concentration from this route is not significant or is markedly reduced when peeling, washing, cooking, and other food preparation processes are taken into consideration (U.S. EPA, 1994d). The decision to not include the irrigation pathway is only an issue when there is medium to heavy irrigation using contaminated water for a radionuclide with a long half-life, a low Kd value, and an insignificant contribution from external exposure. The model also makes a conservative assumption to ignore the decay between harvest and ingestion and any removal during food processing. The model does not include any special calculations for estimating concentrations of 3H and 14C in plants. Such calculations assume that a state of equilibrium exists among the concentrations of 3H and 14C in all environmental media—air, water, food products, and body tissues. This assumption may be overly conservative for a radioactively contaminated site with a finite area, but may be appropriate for an individual pathway, such as soil-to-plant pathway. For these calculations, the 3H concentration in the plant is assumed to be the same as that in the contaminated water to which the plant is exposed. Similarly, the specific activity of 14C in the plant (i.e., pCi/g of 14C per gram of carbon in the plant) is the same as that of the ambient CO2. The model provides a factor, the Contaminated Plant Fraction (CPF), to adjust for the fraction of fruits and vegetables obtained from the contaminated site (assuming that the family living on the site obtains a portion of their fruits and vegetables from uncontaminated sources). The ingestion rate used in the calculation thus represents a total ingestion rate, which, when multiplied by the CPF, gives the ingestion rate of contaminated fruits and vegetables. The CPF is dependent upon the surface area of the contaminated zone in m2, As, and is calculated using the following equation. CPF = As / 4,000 0 As 2,000 m2 CPF = 0.5 As > 2,000 m2
For an area greater than 2,000 m2 (i.e., the default contaminated site surface area), the CPF is set at an upper bound of 0.5 (i.e., site residents acquire no more than one-half of their fruits and vegetables from onsite).
2-23
The factor decreases linearly as the size of the contaminated area decreases below 2,000 m2 (one-half acre).
Equation 5: Screening Level Equation for Ingestion of Radionuclides in Homegrown Produce SSL TR SFp × ( IRvf IRlv ) × 1x10 3 × CPF × TFp × ED
pathway when the size (i.e., area and depth) of the contaminated soil source is known or can be estimated with confidence. Attachment D provides the toxicity criteria and regulatory benchmarks for 60 radionuclides commonly found at NPL sites. These criteria were obtained from HEAST (U.S. EPA, 1995a), and Drinking Water Regulations and Health Advisories (U.S. EPA, 1995c), which are regularly updated. Prior to calculating SSLs at a site, all relevant radionuclide-specific values in Attachment D should be checked against the most recent version of their sources to ensure that they are up to date. Site-specific parameters necessary to calculate SSLs for subsurface soils are listed on Exhibit 11, along with recommended sources and measurement methods. In addition to the soil parameters described in Step 3, other site-specific input parameters include soil moisture, infiltration rate, and aquifer parameters. Guidance for collecting or estimating these other parameters at a site is provided on Exhibit 11 and in Attachment A.
Parameter/Definition (units) TR/target cancer risk (unitless) SFp /produce ingestion slope factor (pCi)-1 IRvf /vegetable and fruit ingestion rate (kg/yr) IRlv /leafy vegetables ingestion rate (kg/yr) 1x10+3/conversion factor (g/kg) CPF/contaminated plant fraction from the site (unitless) TFp/soil-to-plant transfer factor (pCi/g plant per pCi/g soil) ED/exposure duration (yr)
Default 10-6 See Attachment D 42.7 4.66 -0.5 See Attachment C 30
Default values are used for all input parameters in Equation 5 to calculate SSLs for this pathway. With the exception of the contaminated site surface area, As, the amount of data required to derive site-specific values for these parameters makes their collection and use impracticable for calculation of simple site-specific SSLs. Therefore, site-specific data are generally not available for this exposure pathway. The generic SSLs presented in the TBD are recommended for all sites, except for very small sites with As < 2,000 m2 (i.e., < 0.5 acre).
2.5.2 SSL Equations--Subsurface Soils. The Soil Screening Guidance for Radionuclides
addresses only one exposure pathway for subsurface soils: ingestion of ground water contaminated by the migration of contaminants through soil to an underlying potable aquifer. Because the equations developed to calculate SSLs for these pathways assume an infinite source, they can violate mass-balance considerations, especially for small sources. To address this concern, the guidance also includes equations for calculating mass-limit SSLs for this
2-24
Exhibit 11. Site-specific Parameters for Calculating Subsurface SSLs
SSL Pathway Parameter Source Characteristics Source area (A) Source length (L) Source depth Soil Characteristics Soil texture Dry soil bulk density (ρb) Soil moisture content (w) Soil pH Moisture retention exponent (b) Saturated hydraulic conductivity (Ks) Avg. soil moisture content (θw) Meteorological Data Air dispersion factor (Q/C) Hydrogeologic Characteristics (DAF) Hydrogeologic setting Conceptual site model HELP model; Regional estimates Place site in hydrogeologic setting from Aller et al. (1987) for estimation of parameters below (see Attachment A) HELP (Schroeder et al., 1984) may be used for sitespecific infiltration estimates; recharge estimates also may be taken from Aller et al. (1987) or may be estimated from knowledge of local meteorologic and hydrogeologic conditions Aquifer tests (i.e., pump tests, slug tests) preferred; estimates also may be taken from Aller et al. (1987) or Newell et al. (1990) or may be estimated from knowledge of local hydrogeologic conditions Measured on map of site's water table (preferred); estimates also may be taken from Newell et al. (1990) or may be estimated from knowledge of local hydrogeologic conditions Site-specific measurement (i.e., from soil boring logs) preferred; estimates also may be taken from Newell et al. (1990) or may be estimated from knowledge of local hydrogeologic conditions Q/C table (Table 5) Select value corresponding to source area, climatic zone, and city with conditions similar to site Lab measurement Field measurement Lab measurement Field measurement Look-up Look-up Calculated Particle size analysis (Gee & Bauder, 1986) and USDA classification; used to estimate θw & I All soils: ASTM D 2937; shallow soils: ASTM D 1556, ASTM D 2167, ASTM D 2922 ASTM D 2216; used to estimate dry soil bulk density McLean (1982); used to select pH-specific Kd (metals) Attachment A; used to calculate θw Attachment A; used to calculate θw Attachment A Sampling data Sampling data Sampling data Measure total area of contaminated soil Measure length of source parallel to ground water flow Measure depth of contamination or use conservative assumption Migration to ground water Data source Method
Infiltration/recharge (I)
Hydraulic conductivity (K)
Field measurement; Regional estimates
Hydraulic gradient (i)
Field measurement; Regional estimates
Aquifer thickness (d)
Field measurement; Regional estimates
Indicates parameters used in the SSL equations. Indicates parameters/assumptions needed to estimate SSL equation parameters.
2-25
Migration to Ground Water SSLs. The Soil Screening Guidance for Radionuclides uses a simple linear equilibrium soil/water partition equation or a leach test to estimate contaminant release in soil leachate. It also uses a simple water-balance equation to calculate a dilution factor to account for reduction of soil leachate concentration from mixing in an aquifer. The methodology for developing SSLs for the migration to ground water pathway was designed for use during the early stages of a site evaluation when information about subsurface conditions may be limited. Hence, the methodology is based on rather conservative, simplified assumptions about the release and transport of contaminants in the subsurface (Exhibit 12). These assumptions are inherent in the SSL equations and should be reviewed for consistency with the conceptual site model (see Step 2) to determine the applicability of SSLs to the migration to ground water pathway. To calculate SSLs for the migration to ground water pathway, multiply the acceptable ground water concentration by the dilution factor to obtain a target soil leachate concentration. For example, if the dilution factor is 10 and the acceptable ground water concentration is 10 pCi/L, the target soil/water leachate concentration would be 100 pCi/L. Next, the partition equation is used to calculate the total soil concentration (i.e., SSL) corresponding to this soil leachate concentration. Alternatively, if a leach test is used, compare the target soil leachate concentration to extract concentrations from the leach tests. Soil/Water Partition Equation. The soil/water partition equation (Equation 6 ) relates concentrations of contaminants adsorbed to soil organic carbon to soil leachate concentrations in the zone of contamination. It calculates SSLs corresponding to target soil leachate contaminant concentrations (Cw). An adjustment has been added to the equation to relate sorbed concentration in soil to the measured total soil concentration. This adjustment assumes that soil-water and solids are conserved during sampling.
Exhibit 12: Simplifying Assumptions for the SSL Migration to Ground Water Pathway • The source is infinite (i.e., steady-state concentrations will be maintained in ground water over the exposure period of interest) • Contaminants are uniformly distributed throughout the zone of contamination • Soil contamination extends from the surface to the water table (i.e., adsorption sites are filled in the unsaturated zone beneath the area of contamination • There is no chemical or biological degradation in the unsaturated zone • Equations in this document do not account for decay, however an electronic version of these equations will account for decay in the unsaturated zone • Equilibrium soil/water partitioning is instantaneous and linear in the contaminated soil • The receptor well is at the edge of the source (i.e., there is no dilution from recharge downgradient of the site) and is screened within the plume • The aquifer is unconsolidated and unconfined (surficial) • Aquifer properties are homogenous and isotropic • Chelating or complexing agents not present • No facilitated transport (e.g., colloidal transport of inorganic contaminants in aquifer
The use of the soil/water partition equation to calculate SSLs assumes an infinite source (steady-state) of contaminants that extend to the water table. More detailed models may be used to calculate higher SSLs that are still protective in some situations. For example, contaminants at sites with shallow sources, thick unsaturated zones, degradable contaminants, or unsaturated zone characteristics (e.g., clay layers) may attenuate before they reach ground water. Part 3 of the TBD provides information on the use of unsaturated zone models for soil screening. The decision to use such models should be based on balancing the additional investigative and modeling costs required to apply the more complex models against the cost savings that will result from higher SSLs.
2-26
Equation 6 : Soil Screening Level Partitioning Equation for Migration to Ground Water SSL Cw × 1x10 3 × ( Kd θw ρb )
Parameter/Definition (units) SSL/ Screening Level in Soil (pCi/g) Cw/target soil leachate concentration ( pCi/L) 1x10-3/conversion factor (kg/g) Kd/soil-water partition coefficient (L/kg) θw/water-filled soil porosity (Lwater/Lsoil) n/soil porosity (Lpore/Lsoil) ρb/dry soil bulk density (kg/L)
a b
Default MCL a x dilution factor -chemical specific 0.3 1-(ρb /ρs) 1.5
Stabilization/Solidification of CERCLA and RCRA Wastes (U.S. EPA, 1989b) and the EPA SAB’s review of leaching tests (U.S. EPA, 1991c) discuss the application of various leach tests to various waste disposal scenarios. Consult these documents for further information. See Step 3 for guidance on collecting subsurface soil samples that can be used for leach tests. To ensure adequate precision of leach test results, leach tests should be conducted in triplicate. Dilution Factor Model. As soil leachate moves through soil and ground water, contaminant concentrations are attenuated by adsorption and degradation. In the aquifer, dilution by clean ground water further reduces concentrations before contaminants reach receptor points (i.e., drinking water wells). This reduction in concentration can be expressed by a dilution attenuation factor (DAF), defined as the ratio of soil leachate concentration to receptor point concentration. The lowest possible DAF is 1, corresponding to the situation where there is no dilution or attenuation of a contaminant (i.e., when the concentration in the receptor well is equal to the soil leachate concentration). On the other hand, high DAF values correspond to a large reduction in contaminant concentration from the contaminated soil to the receptor well. The Soil Screening Guidance for Radionuclides addresses only one of these dilution-attenuation processes: contaminant dilution in ground water. A simple mixing zone equation derived from a waterbalance relationship (Equation 7) is used to calculate a site-specific dilution factor. Mixing-zone depth is estimated from Equation 8, which relates it to aquifer thickness along with the other parameters from Equation 7. Mixing zone depth should not exceed aquifer thickness (i.e., use aquifer thickness as the upper limit for mixing zone depth). Because of the uncertainty resulting from the wide variability in subsurface conditions that affect contaminant migration in ground water, defaults are not provided for the dilution model equations. Instead, a default DAF of 20 has been selected as protective for contaminated soil sources up to 0.5 acre in size. Analyses using the mass-limit models described in the SSG for chemicals suggest that a DAF of 20 may be protective of larger sources as well; however, this
ρs/soil particle density (kg/L) 2.65 Radionuclide -specific (see Attachment D). See Attachment C.
Leach Test. A leach test may be used instead of the soil/water partition equation. If a leach test is used, compare the target soil leachate concentration (MCL x Dilution Factor) to extract concentrations from the leach tests. In some instances, a leach test may be more useful than the partitioning method, depending on the constituents of concern and the possible presence of RCRA wastes. If this option is chosen, soil parameters are not needed for this pathway. However, a dilution factor must still be calculated. This guidance suggests using the EPA Synthetic Precipitation Leaching Procedure (SPLP, EPA SW-846 Method 1312, U.S. EPA, 1994e). The SPLP was developed to model an acid rain leaching environment and is generally appropriate for a contaminated soil scenario. Like most leach tests, the SPLP may not be appropriate for all situations (e.g., soils contaminated with oily constituents may not yield suitable results). Therefore, apply the SPLP with discretion. EPA is aware that many leach tests are available for application at hazardous waste sites, some of which may be appropriate in specific situations (e.g., the Toxicity Characteristic Leaching Procedure (TCLP) models leaching in a municipal landfill environment). It is beyond the scope of this document to discuss in detail leaching procedures and the appropriateness of their use.
2-27
hypothesis should be evaluated on a site-specific basis. A discussion of the basis for the default DAF and a description of the mass-limit analysis is found in Part 2.6.4 of the TBD. However, since migration to ground water SSLs are most sensitive to the DAF, site-specific dilution factors should be calculated.
Equation 7: Derivation of Dilution Factor DFw 1 K×i×d I×L Default 20 (0.5-acre source)
the source, calculate both standard and mass-limit SSLs, compare them for each radionuclide of concern and select the higher of the two values. Note that Equation 9 requires a site-specific determination of the average depth of contamination in the source. Step 3 provides guidance for conducting subsurface sampling to determine source depth. Where the actual average depth of contamination is uncertain, a conservative estimate should be used (e.g., the maximum possible depth in the unsaturated zone). At many sites, the average water table depth may be used unless there is reason to believe that contamination extends below the water table. In this case SSLs do not apply and further investigation of the source in question is needed.
Equation 9: Mass-Limit Soil Screening Level for Migration to Ground Water
Parameter/Definition (units) DFw/dilution factor (unitless) K/aquifer hydraulic conductivity (m/yr) i/hydraulic gradient (m/m) I/infiltration rate (m/yr) d/mixing zone depth (m) L/source length parallel to ground water flow (m) Equation 8:
SSL
Cw × I × ED × 1x10 3 ρb × d s
Default
Estimation of Mixing Zone Depth
2 0.5
Parameter/Definition (units) SSL/ Soil Screening Level in Soil (pCi/g) Cw/target soil leachate concentration (pCi/L) I/infiltration rate (m/yr) ED/exposure duration (yr) 1x10-3/conversion factor (kg/g) ρb/dry soil bulk density (kg/L)
a
d (0.0112L )
L × I d a × [1 exp ( )] K × i × da
Parameter/Definition (units) d/mixing zone depth (m) L/source length parallel to ground water flow (m) I/infiltration rate (m/yr) K/aquifer hydraulic conductivity (m/yr) i/hydraulic gradient (m/m) da/aquifer thickness (m)
( MCL, )a * dilution factor site-specific 70 -1.5
site-specific ds/depth of source (m) Radionuclide -specific, see Attachment D.
Mass-Limit SSLs. Use of infinite source models to estimate migration to ground water can violate mass balance considerations, especially for small sources. To address this concern, the Soil Screening Guidance includes models for calculating mass-limit SSLs for this pathways (Equation 9) that provide a lower limit to SSLs when the area and depth (i.e., volume) of the source are known or can be estimated reliably.
A mass-limit SSL represents the level of radionuclide in the subsurface that is still protective when the entire volume of contamination leaches over the 30 year exposure duration and the level of radionuclide at the receptor does not exceed the health-based limit. To use mass-limit SSLs, determine the area and depth of
2.5.3 Address Exposure to Multiple Radionuclides . The SSLs generally correspond to
a 10-6 lifetime cancer risk level. The potential for additive effects has not been “built in” to the SSLs through apportionment. While the pathways included in the analysis are considered to represent those a residential setting, SSLs are not calculated for a specific scenario (i.e., SSLs are not summed over a set of pathways). For radionuclides, EPA believes that setting a 10-6 risk level for individual radionuclides and pathways generally will lead to cumulative site risks within the 10-4 to 10-6 risk range for the combinations of radionuclides typically found at NPL sites.
2-28
SSLs and the Use of Surrogate Measurements. For sites with multiple radionuclides, it may be possible to measure just one of the radionuclides and still be able to demonstrate compliance (with the target risk level of 10-6) for the radionuclides present through the use of surrogate measurements. Both time and resources can be saved if the analysis of one radionuclide is simpler than the analysis of the other. For example, using the measured 137Cs concentration as a surrogate for 90Sr reduces the analytical costs because the wet chemistry separations do not have to be performed for 90Sr on every sample. In using one radionuclide to estimate the presence of others, a sufficient number of measurements, spatially separated throughout the EA, should be made to establish a consistent ratio. The number of measurements needed to determine the ratio is selected using the DQO process and based on the chemical, physical, and radiological characteristics of the nuclides and the site. The potential for shifts or variations in the radionuclide ratios means that the surrogate method should be used with caution. Physical or chemical differences between the radionuclides may produce different migration rates, causing the radionuclides to separate and changing the radionuclide ratios. Remediation activities have a reasonable potential to alter the surrogate ratio established prior to remediation. When the ratio is established prior to remediation, additional postremediation samples should be collected to ensure that the data used to establish the ratio are still appropriate and representative of the existing site condition. If these additional post-remediation samples are not consistent with the pre-remediation data, surrogate ratios should be re-established.
In theory, an exposure area would be screened from further investigation when the true mean of the population of radionuclide concentrations falls below the established screening level. However, EPA recognizes that data obtained from sampling and analysis are never perfectly representative and accurate, and that the cost of trying to achieve perfect results would be quite high. Consequently, EPA acknowledges that some uncertainty in data must be tolerated, and focuses on controlling the uncertainty which affects decisions based on those data. Thus, in the Soil Screening Guidance for Radionuclides, EPA has developed an approach for surface soils to minimize the chance of incorrectly deciding to: • • Screen out areas when the correct decision would be to investigate further (Type I error); or Decide to investigate further when the correct decision would be to screen out the area (Type II error).
The approach sets limits on the probabilities of making such decision errors, and acknowledges that there is a range (i.e., gray region) of radionuclide concentrations around the screening level where the variability in the data will make it difficult to determine whether the exposure area average concentration is actually above or below the screening level. The Type I and Type II decision error rates have been set at 5 percent and 20 percent, respectively, and the gray region has been set between one-half and two times the SSL. By specifying the upper edge of the gray region as twice the SSL, it is possible that exposure areas with mean radionuclide concentration values slightly above the SSL may be screened from further study.
2.6
Step 6: Comparing Site Soil Radionuclide Concentrations to Calculated SSLs
Now that the site-specific SSLs have been calculated for the potential radionuclides of concern, compare them with the site radionuclide concentrations. At this point, it is reasonable to review the CSM with the actual site data to confirm its accuracy and the overall applicability of the Soil Screening Guidance for Radionuclides.
2.6.1 Evaluation of Data for Surface Soils. Thus, for surface soils, the radionuclide concentrations in each composite sample from an exposure area are compared to two times the SSL. Under the Soil Screening Guidance DQOs, areas are screened out from further study when radionuclide concentrations in all of the composite samples are less than two times the SSLs. Use of this decision rule (comparing radionuclide concentrations to twice the SSL) is appropriate only when the quantity and quality of data are comparable to the levels discussed in this guidance.
2-29
For existing data sets that may be more limited than those discussed in this guidance, the 95 percent upperconfidence limit on the arithmetic mean of the radionuclide concentrations in surface soils (i.e., the Land method as described in the Supplemental Guidance to RAGS: Calculating the Concentration Term (U.S. EPA, 1992c) should be compared to the SSL. If the 95 percentile on the arithmetic mean is less than the SSL, the exposure area may be screened out. The TBD discusses the strengths and weaknesses of using the Land method for making screening decisions. As an alternative to the Max test, the TBD provides guidance on performing the Sign test when the contaminant is not present in background.
warranted where cumulative risks for current or future land use exceed 1x10-4. The data collected for soil screening are useful in the RI and baseline risk assessment. However, additional data will probably need to be collected during future site investigations. Once the decision has been made to initiate remedial action, the SSLs can then serve as preliminary remediation goals. This process is referenced in Section 1.2 of this document.
FOR FURTHER INFORMATION
More detailed discussions of the technical background and assumptions supporting the development of the Soil Screening Guidance are presented in the Soil Screening Guidance for Radionuclides: Technical Background Document (U.S. EPA, 1999). For additional copies of this guidance document, the Technical Background Document, or other EPA documents, call the National Technical Information Service (NTIS) at (703) 605-6000 or 1-800-553-NTIS (6847). Copies may also be downloaded from the internet at: http://www.epa.gov/superfund/resources/radiation/rad risk.htm.
2.6.2 Evaluation of Data for Subsurface Soils. In this guidance, fewer samples are collected
for subsurface soils than for surface soils; therefore, different decision rules apply. Since subsurface soils are not characterized as well as surface soils, there is less confidence that the concentrations measured are representative of the entire source. Thus, a more conservative approach to screening is warranted. Because it may not be protective to allow for comparison to values above the SSL, mean radionuclide concentrations from each soil boring taken in a source area are compared with the calculated SSLs. Source areas with any mean soil boring radionuclide concentration greater than the SSLs generally warrant further consideration. On the other hand, where the mean soil boring radionuclide concentrations within a source are all less than the SSLs, that source area is generally screened out.
2.7
Step 7: Addressing Areas Identified for Further Study
The radionuclides, exposure pathways, and areas that have been identified for further study become a subject of the RI/FS. The results of the baseline risk assessment conducted as part of the RI/FS will establish the basis for taking remedial action. The threshold for taking action differs from the criteria used for screening. As outlined in Role of the Baseline Risk Assessment in Superfund Remedy Selection Decisions (U.S. EPA, 1991d), remedial action at NPL sites is generally
2-30